The present invention relates generally to the field of solenoids and specifically to double air gap high speed solenoid improvements.
With the advent of electronic fuel injection, there has arisen a need for small, high speed, highly reliable solenoids capable of operating a valve controlling fuel flow into the individual cylinders of an internal combustion engine. Such a solenoid must open at the desired instant and remain open only long enough to allow the precise amount of fuel to be metered into the cylinder of the engine. If the solenoid is not extremely consistent in its operation, dramatic differences in engine fueling will result causing rough running and/or poor fuel economy.
In attempting to make a small, high speed solenoid, it is desirable to have a large coil so as to generate a large magnetic flux while at the same time minimizing the size of the coil to stay within a relatively small package. Further, the pole piece (the fixed core of the solenoid) and the armature (the moveable portion of the solenoid) are generally arranged so that the magnetic flux crosses one air gap between them in the direction of solenoid movement (the operating direction) which causes the attraction which operates the solenoid. The magnetic flux path then returns through a radial air gap which does not contribute to the attractive forces. The strength of the circulating loop of magnetic flux is determined by the coil size, current flow through the coil, magnetic permeability of the core pieces and the magnetic reluctance across the various air gaps. To a certain extent, the small size requirement of fuel injection solenoids works against the use of a large coil and/or a large core to develop large flux flows through the core.
In the interest of both volumetric efficiency and power efficiency a high speed solenoid must develop maximum force which can be shown to correspond to approximately 260 lbs. per square inch of steel area under saturated conditions. This degree of efficiency also is dependent upon minimizing flux fringing, that is, flux lines which do not pass through a working air gap, and upon eliminating the energy loss associated with driving flux through a non-working air gap.
In the past, two-dimensional double air gap solenoids have been utilized to provide an increased operating force in the operating direction without a corresponding increase in flux density. U.S. Pat. No. 3,157,831 issued to W. A. Ray on November 17, 1964 is an example of a two-dimensional double air gap solenoid. A circular coil is wound so as to provide a toroidal flux path. The pole piece of laminated plate construction has three legs, center leg 3 which extends into the coil and outer leg 2 and 4 which extend on the outer portion of the core. The armature 19 is also laminated and a center leg 23 extends into the coil 14 and outer legs 21 and 22 extend outside the core. Upon energization of the electromagnetic coil, the center legs of the core and armature attract each other as do the outer two legs of each structure. The air gaps between the legs of the armature and the legs of the pole piece extend in the operating direction of the solenoid such that attractive forces generated by the flux passing through an air gap are all in the desired operating direction. This is a distinct improvement over prior art solenoids which generally included a radial air gap in the return magnetic flux path. Such a radial air gap would also cause sideways forces on the armature increasing the wear of armature bushings and other components. Furthermore, this sideways attractive force is not in the desired operating direction and therefore is "wasted" as far as the solenoid operation is concerned.
Difficulties with the two-dimensional double air gap solenoids include the failure to maximize flux passage as a result of current flow in the coil in directions other than the two-dimensional plane. This failure results in a loss of efficiency. Additionally, although eddy current generation is minimized in two-dimensional solenoids by the use of laminated plates making up the armature and the core, the use of laminated cores does not lend itself to the construction of cylindrical, closed construction as is preferable for better volumetric efficiency and the exclusion of contaminating particles.
Also, one characteristic of many solenoids is that given a fixed operating current through the coil, the attractive force between the pole piece and the armature varies as the inverse exponential of the distance between the two. Consequently, if a high initial force is needed to accelerate the armature to a specific desired traveling speed, a short air gap is necessary. However, the use of a short air gap also limits the operating travel of the solenoid to a similar short distance. In some solenoids, complex lever arms and the like have been utilized in an attempt to obtain a longer stroke and yet still operate with the pole and armature spacing very small.